Network Working Group C. Holmberg
Internet-Draft Ericsson
Updates: 5763,7345 (if approved) R. Shpount
Intended status: Standards Track TurboBridge
Expires: May 2, 2018 October 29, 2017
Session Description Protocol (SDP) Offer/Answer Considerations for
Datagram Transport Layer Security (DTLS) and Transport Layer Security
(TLS)
draft-ietf-mmusic-dtls-sdp-32.txt
Abstract
This document defines the Session Description Protocol (SDP) offer/
answer procedures for negotiating and establishing a Datagram
Transport Layer Security (DTLS) association. The document also
defines the criteria for when a new DTLS association must be
established. The document updates RFC 5763 and RFC 7345, by
replacing common SDP offer/answer procedures with a reference to this
specification.
This document defines a new SDP media-level attribute, 'tls-id'.
This document also defines how the 'tls-id' attribute can be used for
negotiating and establishing a Transport Layer Security (TLS)
connection, in conjunction with the procedures in RFC 4145 and RFC
8122.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 2, 2018.
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Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Establishing a new DTLS Association . . . . . . . . . . . . . 4
3.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Change of Local Transport Parameters . . . . . . . . . . 5
3.3. Change of ICE ufrag value . . . . . . . . . . . . . . . . 5
4. SDP tls-id Attribute . . . . . . . . . . . . . . . . . . . . 5
5. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 7
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Generating the Initial SDP Offer . . . . . . . . . . . . 9
5.3. Generating the Answer . . . . . . . . . . . . . . . . . . 10
5.4. Offerer Processing of the SDP Answer . . . . . . . . . . 11
5.5. Modifying the Session . . . . . . . . . . . . . . . . . . 11
6. ICE Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. TLS Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. SIP Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. RFC Updates . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.2. Update to RFC 5763 . . . . . . . . . . . . . . . . . . . 14
9.2.1. Update to section 1 . . . . . . . . . . . . . . . . . 14
9.2.2. Update to section 5 . . . . . . . . . . . . . . . . . 15
9.2.3. Update to section 6.6 . . . . . . . . . . . . . . . . 16
9.2.4. Update to section 6.7.1 . . . . . . . . . . . . . . . 16
9.3. Update to RFC 7345 . . . . . . . . . . . . . . . . . . . 17
9.3.1. Update to section 4 . . . . . . . . . . . . . . . . . 17
9.3.2. Update to section 5.2.1 . . . . . . . . . . . . . . . 17
9.3.3. Update to section 10.1 . . . . . . . . . . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 19
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14. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
14.1. Normative References . . . . . . . . . . . . . . . . . . 24
14.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
[RFC5763] defines Session Description Protocol (SDP) offer/answer
procedures for Secure Realtime Transport Protocol Using Datagram
Transport Layer Security (DTLS-SRTP). [RFC7345] defines SDP offer/
answer procedures for UDP Transport Layer over Datagram Transport
Layer Security (UDPTL-DTLS). This specification defines general
offer/answer procedures for DTLS, based on the procedures in
[RFC5763]. Other specifications, defining specific DTLS usages, can
then reference this specification, in order to ensure that the DTLS
aspects are common among all usages. Having common procedures is
essential when multiple usages share the same DTLS association
[I-D.ietf-mmusic-sdp-bundle-negotiation]. The document updates
[RFC5763] and [RFC7345], by replacing common SDP offer/answer
procedures with a reference to this specification.
NOTE: Since the publication of [RFC5763], [RFC4474] has been
obsoleted by [I-D.ietf-stir-rfc4474bis]. The updating of the
references (and the associated procedures) within [RFC5763] is
outside the scope of this document. However, implementers of
[RFC5763] applications are encouraged to implement
[I-D.ietf-stir-rfc4474bis] instead of [RFC4474].
As defined in [RFC5763], a new DTLS association MUST be established
when transport parameters are changed. Transport parameter change is
not well defined when Interactive Connectivity Establishment (ICE)
[I-D.ietf-ice-rfc5245bis] is used. One possible way to determine a
transport change is based on ufrag [I-D.ietf-ice-rfc5245bis] change,
but the ufrag value is changed both when ICE is negotiated and when
ICE restart [I-D.ietf-ice-rfc5245bis] occurs. These events do not
always require a new DTLS association to be established, but
previously there was no way to explicitly indicate in an SDP offer or
answer whether a new DTLS association is required. To solve that
problem, this document defines a new SDP attribute, 'tls-id'. The
pair of SDP 'tls-id' attribute values (the attribute values of the
offerer and the answerer) uniquely identifies the DTLS association.
Providing a new value of the 'tls-id' attribute in an SDP offer or
answers can be used to indicate whether a new DTLS association is to
be established.
The SDP 'tls-id' attribute can be specified when negotiating a
Transport Layer Security (TLS) connection, using the procedures in
this document in conjunction with the procedures in [RFC5763] and
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[RFC8122]. The unique combination of SDP 'tls-id' attribute values
can be used to identity the negotiated TLS connection. The unique
value can be used, for example, within TLS protocol extensions to
differentiate between multiple TLS connections and correlate those
connections with specific offer/answer exchanges. The TLS specific
considerations are described in Section 7.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Establishing a new DTLS Association
3.1. General
A new DTLS association must be established between two endpoints
after a successful SDP offer/answer exchange in the following cases:
o The negotiated DTLS setup roles change; or
o One or more fingerprint values are modified, added or removed in
either an SDP offer or answer; or
o The intent to establish a new DTLS association is explicitly
signaled using SDP, by changing the value of the SDP 'tls-id'
attribute defined in this document;
NOTE: The first two items above are based on the procedures in
[RFC5763]. This specification adds the support for explicit
signaling using the SDP 'tls-id' attribute.
A new DTLS association can only be established as a result of the
successful SDP offer/answer exchange. Whenever an entity determines
that a new DTLS association is required, the entity MUST initiate an
SDP offer/answer exchange, following the procedures in Section 5.
The sections below describe typical cases where a new DTLS
association needs to be established.
In this document, a "new DTLS association" between two endpoints
refers to either an initial DTLS association (when no DTLS
association is currently established between the endpoints) or an
DTLS association replacing a previously established DTLS association.
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3.2. Change of Local Transport Parameters
If an endpoint modifies its local transport parameters (address and/
or port), and if the modification requires a new DTLS association,
the endpoint MUST change its local SDP 'tls-id' attribute value (see
Section 4).
If the underlying transport protocol prohibits a DTLS association
from spanning multiple 5-tuples (transport/source address/source
port/destination address/destination port), and if the 5-tuple is
changed, the endpoint MUST change its local SDP 'tls-id' attribute
value (see Section 4). An example of such a case is when DTLS is
carried over the Stream Control Transmission Protocol (SCTP), as
described in [RFC6083].
3.3. Change of ICE ufrag value
If an endpoint uses ICE, and modifies a local ufrag value, and if the
modification requires a new DTLS association, the endpoint MUST
change its local SDP 'tls-id' attribute value (see Section 4).
4. SDP tls-id Attribute
The pair of SDP 'tls-id' attribute values (the attribute values of
the offerer and the answerer) uniquely identifies the DTLS
association or TLS connection.
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Name: tls-id
Value: tls-id-value
Usage Level: media
Charset Dependent: no
Default Value: N/A
Syntax:
tls-id-value = 20*255(tls-id-char)
tls-id-char = ALPHA / DIGIT / "+" / "/" / "-" / "_"
Example:
a=tls-id:abc3de65cddef001be82
Every time an endpoint requests to establish a new DTLS association,
the endpoint MUST generate a new local 'tls-id' attribute value. A
non-changed local 'tls-id' attribute value, in combination with non-
changed fingerprints, indicates that the endpoint intends to reuse
the existing DTLS association.
The 'tls-id' attribute value MUST be generated using a strong random
function and include at least 120 bits of randomness.
No default value is defined for the SDP 'tls-id' attribute.
Implementations that wish to use the attribute MUST explicitly
include it in SDP offers and answers. If an offer or answer does not
contain a 'tls-id' attribute (this could happen if the offerer or
answerer represents an existing implementation that has not been
updated to support the 'tls-id' attribute), unless there is another
mechanism to explicitly indicate that a new DTLS association is to be
established, a modification of one or more of the following
characteristics MUST be treated as an indication that an endpoint
wants to establish a new DTLS association:
o DTLS setup role; or
o fingerprint set; or
o local transport parameters
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NOTE: A modification of the ufrag value is not treated as an
indication that an endpoint wants to establish a new DTLS assocation.
In order to indicate that a new DTLS association is to be
established, one or more of the characteristics listed above have to
be modified.
The mux category [I-D.ietf-mmusic-sdp-mux-attributes] for the 'tls-
id' attribute is 'IDENTICAL', which means that the attribute value
applies to all media descriptions being multiplexed
[I-D.ietf-mmusic-sdp-bundle-negotiation]. However, as described in
[I-D.ietf-mmusic-sdp-bundle-negotiation], in order to avoid
duplication the attribute is only associated with the "m=" line
representing the offerer/answerer BUNDLE-tag.
For RTP-based media, the 'tls-id' attribute applies to the whole
associated media description. The attribute MUST NOT be defined per
source (using the SDP 'ssrc' attribute [RFC5576]).
The SDP offer/answer [RFC3264] procedures associated with the
attribute are defined in Section 5.
5. SDP Offer/Answer Procedures
5.1. General
This section defines the generic SDP offer/answer procedures for
negotiating a DTLS association. Additional procedures (e.g.,
regarding usage of specific SDP attributes etc.) for individual DTLS
usages (e.g., DTLS-SRTP) are outside the scope of this specification,
and need to be specified in a usage specific specification.
NOTE: The procedures in this section are generalizations of
procedures first specified in DTLS-SRTP [RFC5763], with the addition
of usage of the SDP 'tls-id' attribute. That document is herein
updated to make use of these new procedures.
The procedures in this section apply to an SDP media description
("m=" line) associated with DTLS-protected media/data.
When an offerer or answerer indicates that it wants to establish a
new DTLS association, it needs to make sure that media packets
associated with any previously established DTLS association and the
new DTLS association can be de-multiplexed. In case of an ordered
transport (e.g., SCTP) this can be done simply by sending packets for
the new DTLS association after all packets associated with a
previously established DTLS association has been sent. In case of an
unordered transport, such as UDP, packets associated with a
previously established DTLS association can arrive after the answer
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SDP was received and after the first packets associated with the new
DTLS association were received. The only way to de-multiplex packets
associated with with a previously established DTLS association and
the new DTLS association is on the basis of the 5-tuple. Because of
this, if an unordered transport is used for the DTLS association, a
new 3-tuple (transport/source address/source port) MUST be allocated
by at least one of the endpoints so that DTLS packets can be de-
multiplexed.
When an offerer needs to establish a new DTLS association, and if an
unordered transport (e.g., UDP) is used, the offerer MUST allocate a
new 3-tuple for the offer in such a way that the offerer can
disambiguate any packets associated with the new DTLS association
from any packets associated with any other DTLS association. This
typically means using a local address and/or port, or a set of ICE
candidates (see Section 6), which were not recently used for any
other DTLS association.
When an answerer needs to establish a new DTLS association, if an
unordered transport is used, and if the offerer did not allocate a
new 3-tuple, the answerer MUST allocate a new 3-tuple for the answer
in such a way that it can disambiguate any packets associated with
the new DTLS association from any packets associated with any other
DTLS association. This typically means using a local address and/or
port, or a set of ICE candidates (see Section 6), which were not
recently used for any other DTLS association.
In order to negotiate a DTLS association, the following SDP
attributes are used:
o The SDP 'setup' attribute, defined in [RFC4145], is used to
negotiate the DTLS roles;
o The SDP 'fingerprint' attribute, defined in [RFC8122], is used to
provide one or more fingerprint values; and
o The SDP 'tls-id' attribute, defined in this specification, is used
to identity the DTLS association.
This specification does not define the usage of the SDP 'connection'
attribute [RFC4145] for negotiating a DTLS association. However, the
attribute MAY be used if the DTLS association is used together with
another protocol (e.g., SCTP or TCP) for which the usage of the
attribute has been defined.
Unlike for TCP and TLS connections, endpoints MUST NOT use the SDP
'setup' attribute 'holdconn' value when negotiating a DTLS
association.
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Endpoints MUST support the hash functions as defined in [RFC8122].
The certificate received during the DTLS handshake [RFC6347] MUST
match a certificate fingerprint received in SDP 'fingerprint'
attributes according to the procedures defined in [RFC8122]. If
fingerprints do not match the hashed certificate, then an endpoint
MUST tear down the media session immediately (see [RFC8122]).
SDP offerers and answerers might reuse certificates across multiple
DTLS associations, and provide identical fingerprint values for each
DTLS association. The combination of the SDP 'tls-id' attribute
values of the SDP offerer and answerer identifies each individual
DTLS association.
NOTE: There are cases where the SDP 'tls-id' attribute value
generated by the offerer will end up being used for multiple DTLS
associations. For that reason the combination of the attribute
values of the offerer and answerer is needed in order to identity a
DTLS association. An example of such case is where the offerer sends
an updated offer (Section 5.5), without modifying its attribute
value, but the answerer determines that a new DTLS association is to
be created. The answerer will generate a new local attribute value
for the new DTLS association (Section 5.3), while the offerer will
use the same attribute value that it used for the current
association. Another example is when the Session Initiation Protocol
(SIP) [RFC3261] is used for signalling, and an offer is forked to
multiple answerers. The attribute value generated by the offerer
will be used for DTLS associations established by each answerer.
5.2. Generating the Initial SDP Offer
When an offerer sends the initial offer, the offerer MUST insert an
SDP 'setup' attribute [RFC4145] with an 'actpass' attribute value,
and one or more SDP 'fingerprint' attributes according to the
procedures in [RFC8122]. In addition, the offerer MUST insert in the
offer an SDP 'tls-id' attribute with a unique attribute value.
As the offerer inserts the SDP 'setup' attribute with an 'actpass'
attribute value, the offerer MUST be prepared to receive a DTLS
ClientHello message [RFC6347] (if a new DTLS association is
established by the answerer) from the answerer before the offerer
receives the SDP answer.
If the offerer receives a DTLS ClientHello message, and a DTLS
association is established, before the offerer receives the SDP
Answer carrying the fingerprint associated with the DTLS association,
any data received on the DTLS association before the fingerprint MUST
be considered coming from an unverified source. The processing of
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such data, and sending of data by the offerer to the unverified
source, is outside the scope of this document.
5.3. Generating the Answer
When an answerer sends an answer, the answerer MUST insert in the
answer an SDP 'setup' attribute according to the procedures in
[RFC4145], and one or more SDP 'fingerprint' attributes according to
the procedures in [RFC8122]. If the answerer determines, based on
the criteria specified in Section 3.1, that a new DTLS association is
to be established, the answerer MUST insert in the associated answer
an SDP 'tls-id' attribute with a new unique attribute value. Note
that the offerer and answerer generate their own local 'tls-id'
attribute values, and the combination of both values identify the
DTLS association.
If the answerer receives an offer that requires establishment of a
new DTLS association, and if the answerer does not accept the
establishment of a new DTLS association, the answerer MUST reject the
"m=" lines associated with the suggested DTLS association [RFC3264].
If an answerer receives an offer that does not require the
establishment of a new DTLS association, and if the answerer
determines that a new DTLS association is not to be established, the
answerer MUST insert an SDP 'tls-id' attribute with the previously
assigned attribute value in the associated answer. In addition, the
answerer MUST insert an SDP 'setup' attribute with an attribute value
that does not change the previously negotiated DTLS roles, and one or
more SDP 'fingerprint' attributes values that do not change the
previously sent fingerprint set, in the associated answer.
If the answerer receives an offer that does not contain an SDP 'tls-
id' attribute, the answerer MUST NOT insert a 'tls-id' attribute in
the answer.
If a new DTLS association is to be established, and if the answerer
inserts an SDP 'setup' attribute with an 'active' attribute value in
the answer, the answerer MUST initiate a DTLS handshake [RFC6347]) by
sending a DTLS ClientHello message towards the offerer.
Even though an offerer is required to insert an 'SDP' setup attribute
with an 'actpass' attribute value in initial offers (Section 5.2) and
subsequent offers (Section 5.5), the answerer MUST be able to receive
initial and subsequent offers with other attribute values, in order
to be backward compatible with older implementations that might
insert other attribute values in initial and subsequent offers.
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5.4. Offerer Processing of the SDP Answer
When an offerer receives an answer that establishes a new DTLS
association based on criteria defined in Section 3.1, and if the
offerer becomes DTLS client (based on the value of the SDP 'setup'
attribute value [RFC4145]), the offerer MUST establish a DTLS
association. If the offerer becomes DTLS server, it MUST wait for
the answerer to establish the DTLS association.
If the offerer indicated a desire to reuse an existing DTLS
association and the answerer does not request the establishment of a
new DTLS association, the offerer will continue to use the previously
established DTLS association.
A new DTLS association can be established based on changes in either
an SDP offer or answer. When communicating with legacy endpoints, an
offerer can receive an answer that includes the same fingerprint set
and setup role. A new DTLS association will still be established if
such an answer was received as a response to an offer which requested
the establishment of a new DTLS association, as the transport
parameters would have been changed in the offer.
5.5. Modifying the Session
When an offerer sends a subsequent offer, and if the offerer wants to
establish a new DTLS association, the offerer MUST insert an SDP
'setup' attribute [RFC4145] with an 'actpass' attribute value, and
one or more SDP 'fingerprint' attributes according to the procedures
in [RFC8122]. In addition, the offerer MUST insert in the offer an
SDP 'tls-id' attribute with a new unique attribute value.
When an offerer sends a subsequent offer, and the offerer does not
want to establish a new DTLS association, and if a previously
established DTLS association exists, the offerer MUST insert an SDP
'setup' attribute with an 'actpass' attribute value, and one or more
SDP 'fingerprint' attributes with attribute values that do not change
the previously sent fingerprint set, in the offer. In addition, the
offerer MUST insert an SDP 'tls-id' attribute with the previously
assigned attribute value in the offer.
NOTE: When a new DTLS association is being established, each endpoint
needs to be prepared to receive data on both the new and old DTLS
associations as long as both are alive.
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6. ICE Considerations
When the Interactive Connectivity Establishment (ICE) mechanism
[I-D.ietf-ice-rfc5245bis] is used, the ICE connectivity checks are
performed before the DTLS handshake begins. Note that if aggressive
nomination mode is used, multiple candidate pairs may be marked valid
before ICE finally converges on a single candidate pair.
NOTE: Aggressive nomination has been deprecated from ICE, but must
still be supported for backwards compatibility reasons
[I-D.ietf-ice-rfc5245bis].
When a new DTLS association is established over an unordered
transport, in order to disambiguate any packets associated with the
newly established DTLS association, at least one of the endpoints
MUST allocate a completely new set of ICE candidates which were not
recently used for any other DTLS association. This means the
answerer cannot initiate a new DTLS association unless the offerer
initiated ICE restart [I-D.ietf-ice-rfc5245bis]. If the answerer
wants to initiate a new DTLS association, it needs to initiate an ICE
restart and a new offer/answer exchange on its own. However, an ICE
restart does not by default require a new DTLS association to be
established.
NOTE: Simple Traversal of the UDP Protocol through NAT (STUN) packets
are sent directly over UDP, not over DTLS. [RFC7983] describes how
to demultiplex STUN packets from DTLS packets and SRTP packets.
Each ICE candidate associated with a component is treated as being
part of the same DTLS association. Therefore, from a DTLS
perspective it is not considered a change of local transport
parameters when an endpoint switches between those ICE candidates.
7. TLS Considerations
The procedures in this document can also be used for negotiating and
establishing a TLS connection, with the restriction described below.
As specified in [RFC4145], the SDP 'connection' attribute is used to
indicate whether to establish a new TLS connection. An offerer and
answerer MUST ensure that the 'connection' attribute value and the
'tls-id' attribute value does not cause a conflict regarding whether
a new TLS connection is to be established or not.
NOTE: Even though the SDP 'connection' attribute can be used to
indicate whether a new TLS connection is to be established, the
unique combination of SDP 'tls-id' attribute values can be used to
identity a TLS connection. The unique value can be used e.g., within
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TLS protocol extensions to differentiate between multiple TLS
connections and correlate those connections with specific offer/
answer exchanges. One such extension is defined in
[I-D.ietf-mmusic-sdp-uks].
If an offerer or answerer inserts an SDP 'connection' attribute with
a 'new' value in the offer/answer and also inserts an SDP 'tls-id'
attribute, the value of tls-id' attribute MUST be new and unique.
If an offerer or answerer inserts an SDP 'connection' attribute with
a 'existing' value in the offer/answer, if a previously established
TLS connection exists, and if the offerer/answerer previously
inserted an SDP 'tls-id' attribute associated with the same TLS
connection in an offer/answer, the offerer/answerer MUST also insert
an SDP 'tls-id' attribute with the previously assigned value in the
offer/answer.
If an offerer or answerer receives an offer/answer with conflicting
attribute values, the offerer/answerer MUST process the offer/answer
as misformed.
An endpoint MUST NOT make assumptions regarding the support of the
SDP 'tls-id' attribute by the peer. Therefore, to avoid ambiguity,
both offerers and answerers MUST always use the 'connection'
attribute in conjunction with the 'tls-id' attribute.
NOTE: As defined in [RFC4145], if the SDP 'connection' attribute is
not explicitly present, the implicit default value is 'new'.
The SDP example below is based on the example in section 3.4 of
[RFC8122], with the addition of the SDP 'tls-id' attribute.
m=image 54111 TCP/TLS t38
c=IN IP4 192.0.2.2
a=tls-id:abc3de65cddef001be82
a=setup:passive
a=connection:new
a=fingerprint:SHA-256 \
12:DF:3E:5D:49:6B:19:E5:7C:AB:4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF: \
3E:5D:49:6B:19:E5:7C:AB:4A:AD
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
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8. SIP Considerations
When the Session Initiation Protocol (SIP) [RFC3261] is used as the
signal protocol for establishing a multimedia session, dialogs
[RFC3261] might be established between the caller and multiple
callees. This is referred to as forking. If forking occurs,
separate DTLS associations will be established between the caller and
each callee.
When forking occurs, an SDP offerer can receive DTLS ClientHello
messages and SDP answerers from multiple remote locations. Because
of this, the offerer might have to wait for multiple SDP answers
(from different remote locations) until it receives a certificate
fingerprint that matches the certificate associated with a specific
DTLS handshake. The offerer MUST NOT declare a fingerprint mismatch
until it determines that it will not receive SDP answers from any
additional remote locations.
It is possible to send an INVITE request which does not contain an
SDP offer. Such an INVITE request is often referred to as an 'empty
INVITE', or an 'offer-less INVITE'. The receiving endpoint will
include the SDP offer in a response to the request. When the
endpoint generates such SDP offer, if a previously established DTLS
association exists, the offerer MUST insert an SDP 'tls-id'
attribute, and one or more SDP 'fingerprint' attributes, with
previously assigned attribute values. If a previously established
DTLS association did not exist, the offer MUST be generated based on
the same rules as a new offer (see Section 5.2). Regardless of the
previous existence of a DTLS association, the SDP 'setup' attribute
MUST be included according to the rules defined in [RFC4145].
Furthermore, if ICE is used, according to the third party call
control considerations described in [I-D.ietf-mmusic-ice-sip-sdp],
ICE restart MUST be initiated.
9. RFC Updates
9.1. General
This section updates specifications that use DTLS-protected media, in
order to reflect the procedures defined in this specification.
9.2. Update to RFC 5763
9.2.1. Update to section 1
The reference to [RFC4572] is replaced with a reference to [RFC8122].
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9.2.2. Update to section 5
The text in section 5 (Establishing a Secure Channel) is modified by
replacing generic SDP offer/answer procedures for DTLS with a
reference to this specification:
NEW TEXT:
The two endpoints in the exchange present their identities as part of
the DTLS handshake procedure using certificates. This document uses
certificates in the same style as described in "Connection-Oriented
Media Transport over the Transport Layer Security (TLS) Protocol in
the Session Description Protocol (SDP)" [RFC8122].
If self-signed certificates are used, the content of the
subjectAltName attribute inside the certificate MAY use the uniform
resource identifier (URI) of the user. This is useful for debugging
purposes only and is not required to bind the certificate to one of
the communication endpoints. The integrity of the certificate is
ensured through the fingerprint attribute in the SDP.
The generation of public/private key pairs is relatively expensive.
Endpoints are not required to generate certificates for each session.
The offer/answer model, defined in [RFC3264], is used by protocols
like the Session Initiation Protocol (SIP) [RFC3261] to set up
multimedia sessions.
When an endpoint wishes to set up a secure media session with another
endpoint, it sends an offer in a SIP message to the other endpoint.
This offer includes, as part of the SDP payload, a fingerprint of
a certificate that the endpoint wants to use. The endpoint SHOULD
send the SIP message containing the offer to the offerer's SIP proxy
over an integrity protected channel. The proxy SHOULD add an
Identity header field according to the procedures outlined in
[RFC4474]. When the far endpoint receives the SIP message, it can
verify the identity of the sender using the Identity header field.
Since the Identity header field is a digital signature across several
SIP header fields, in addition to the body of the SIP message, the
receiver can also be certain that the message has not been tampered
with after the digital signature was applied and added to the SIP
message.
The far endpoint (answerer) may now establish a DTLS association with
the offerer. Alternately, it can indicate in its answer that the
offerer is to initiate the DTLS association. In either case, mutual
DTLS certificate-based authentication will be used. After completing
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the DTLS handshake, information about the authenticated identities,
including the certificates, are made available to the endpoint
application. The answerer is then able to verify that the offerer's
certificate used for authentication in the DTLS handshake can be
associated to a certificate fingerprint contained in the offer in
the SDP. At this point, the answerer may indicate to the end user
that the media is secured. The offerer may only tentatively accept
the answerer's certificate since it may not yet have the answerer's
certificate fingerprint.
When the answerer accepts the offer, it provides an answer back to
the offerer containing the answerer's certificate fingerprint. At
this point, the offerer can accept or reject the peer's certificate
and the offerer can indicate to the end user that the media is
secured.
Note that the entire authentication and key exchange for securing
the media traffic is handled in the media path through DTLS. The
signaling path is only used to verify the peers' certificate
fingerprints.
The offerer and answerer MUST follow the SDP offer/answer procedures
defined in [RFCXXXX].
9.2.3. Update to section 6.6
The text in section 6.6 (Session Modification) is modified by
replacing generic SDP offer/answer procedures for DTLS with a
reference to this specification:
NEW TEXT:
Once an answer is provided to the offerer, either endpoint MAY
request a session modification that MAY include an updated offer.
This session modification can be carried in either an INVITE or
UPDATE request. The peers can reuse an existing DTLS association,
or establish a new one, following the procedures in [RFCXXXX].
9.2.4. Update to section 6.7.1
The text in section 6.7.1 (ICE Interaction) is modified by replacing
the ICE procedures with a reference to this specification:
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NEW TEXT:
The Interactive Connectivity Establishment (ICE)
[I-D.ietf-ice-rfc5245bis] considerations for DTLS-protected media
are described in [RFCXXXX].
9.3. Update to RFC 7345
9.3.1. Update to section 4
The subsections (4.1.-4.5.) in section 4 (SDP Offerer/Answerer
Procedures) are removed, and replaced with the new text below:
NEW TEXT:
An endpoint (i.e., both the offerer and the answerer) MUST create an
SDP media description ("m=" line) for each UDPTL-over-DTLS media
stream and MUST assign a UDP/TLS/UDPTL value (see Table 1) to the
"proto" field of the "m=" line.
The offerer and answerer MUST follow the SDP offer/answer procedures
defined in [RFCXXXX] in order to negotiate the DTLS association
associated with the UDPTL-over-DTLS media stream. In addition,
the offerer and answerer MUST use the SDP attributes defined for
UDPTL over UDP, as defined in [ITU.T38.2010].
9.3.2. Update to section 5.2.1
The text in section 5.2.1 (ICE Usage) is modified by replacing the
ICE procedures with a reference to this specification:
NEW TEXT:
The Interactive Connectivity Establishment (ICE)
[I-D.ietf-ice-rfc5245bis] considerations for DTLS-protected media
are described in [RFCXXXX].
[RFC EDITOR NOTE: Throughout the document, please replace RFCXXXX
with the RFC number of this document.]
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9.3.3. Update to section 10.1
A reference to [RFC8122] is added to section 10.1 (Normative
References):
NEW TEXT:
[RFC8122] Lennox, J. and C. Holmberg, "Connection-Oriented Media
Transport over the Transport Layer Security (TLS) Protocol
in the Session Description Protocol (SDP)", RFC 8122,
DOI 10.17487/RFC8122, March 2017,
.
10. Security Considerations
This specification does not modify the security considerations
associated with DTLS, or the SDP offer/answer mechanism. In addition
to the introduction of the SDP 'tls-id' attribute, the specification
simply clarifies the procedures for negotiating and establishing a
DTLS association.
This specification does not modify the actual TLS connection setup
procedures. The SDP 'tls-is' attribute as such cannot be used to
correlate an SDP Offer/Answer exchange with a TLS connection setup.
Thus, this draft does not introduce new security considerations
related to correlating an SDP Offer/Answer exchange with a TLS
connection setup.
11. IANA Considerations
This document updates the "Session Description Protocol Parameters"
registry as specified in Section 8.2.2 of [RFC4566]. Specifically,
it adds the SDP 'tls-id' attribute to the table for SDP media level
attributes.
Attribute name: tls-id
Type of attribute: media-level
Subject to charset: no
Purpose: Indicates whether a new DTLS association or TLS connection
is to be established/re-established.
Appropriate Values: see Section 4
Contact name: Christer Holmberg
Mux Category: IDENTICAL
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12. Acknowledgements
Thanks to Justin Uberti, Martin Thomson, Paul Kyzivat, Jens Guballa,
Charles Eckel, Gonzalo Salgueiro and Paul Jones for providing
comments and suggestions on the document. Ben Campbell performed an
AD review. Paul Kyzivat performed a gen-art review.
13. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing]
Changes from draft-ietf-mmusic-sdp-dtls-31
o Changes based on IESG comments from Eric R
Changes from draft-ietf-mmusic-sdp-dtls-30
o Changes based on IESG comments from Mirja K
Changes from draft-ietf-mmusic-sdp-dtls-29
o Removal of ufrag value change as a trigger for a new DTLS
association
Changes from draft-ietf-mmusic-sdp-dtls-28
o Changes based on IESG review by Adam Roach, Eric Rescorla, Alexey
Melnikov and Mirja Kuhlewind:
o - Document title changed
o - Transport Protocol Considerations section removed
o - Additional text to Security Considerations section
o - Editorial changes
Changes from draft-ietf-mmusic-sdp-dtls-27
o Reference fixes based on Gen-ART review by Paul Kyzivat.
Changes from draft-ietf-mmusic-sdp-dtls-26
o Editorial fixes based on Gen-ART review by Paul Kyzivat.
Changes from draft-ietf-mmusic-sdp-dtls-25
o Minor editorial nits.
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Changes from draft-ietf-mmusic-sdp-dtls-24
o Changes based on 2nd WGLC comments from Roman S and Martin T:
o - RFC update structure shortened (old text removed).
o - Guidance regarding receiving ClientHello before SDP answer
added.
o - Additional SIP considerations regarding forking.
o - SDP setup attribute value restriction in initial and subsequent
offers based on comment from Ekr.
Changes from draft-ietf-mmusic-sdp-dtls-23
o Editorial change to make it clear that the document does not
modify the procedures in RFC 8122.
Changes from draft-ietf-mmusic-sdp-dtls-22
o Support for TLS added.
o Editorial changes based on sec-dir review by Rich Salz.
o Editorial changes based on gen-art review by Paul Kyzivat.
o Editorial changes based on ops-dir review by Carlos Pignataro.
Changes from draft-ietf-mmusic-sdp-dtls-21
o Changes based on AD review by Ben Campbell.
o (https://www.ietf.org/mail-archive/web/mmusic/current/
msg17707.html)
Changes from draft-ietf-mmusic-sdp-dtls-20
o Change to length and randomness of tls-id attribute value.
Changes from draft-ietf-mmusic-sdp-dtls-19
o Change based on comment from Roman.
Changes from draft-ietf-mmusic-sdp-dtls-18
o Changes based on comments from Flemming.
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o - Change in tls-id value definition.
o - Editorial fixes.
Changes from draft-ietf-mmusic-sdp-dtls-17
o Reference fix.
Changes from draft-ietf-mmusic-sdp-dtls-16
o Editorial changes based on 2nd WGLC comments from Christian Groves
and Nevenka Biondic.
Changes from draft-ietf-mmusic-sdp-dtls-15
o tls-id attribute value made globally unique
Changes from draft-ietf-mmusic-sdp-dtls-14
o Changes based on comments from Flemming:
o - Additional dtls-is clarifications
o - Editorial fixes
Changes from draft-ietf-mmusic-sdp-dtls-13
o Text about the updated RFCs added to Abstract and Introduction
o Reference to RFC 5763 removed from section 6 (ICE Considerations)
o Reference to RFC 5763 removed from section 8 (SIP Considerations)
Changes from draft-ietf-mmusic-sdp-dtls-12
o "unreliable" changed to "unordered"
Changes from draft-ietf-mmusic-sdp-dtls-11
o Attribute name changed to tls-id
o Additional text based on comments from Roman Shpount.
Changes from draft-ietf-mmusic-sdp-dtls-10
o Modified document to use tls-id instead of dtls-connection
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o Changes are based on comments from Eric Rescorla, Justin Uberti,
and Paul Kyzivat.
Changes from draft-ietf-mmusic-sdp-dtls-08
o Offer/Answer section modified in order to allow sending of
multiple SDP 'fingerprint' attributes.
o Terminology made consistent: 'DTLS connection' replaced with 'DTLS
association'.
o Editorial changes based on comments from Paul Kyzivat.
Changes from draft-ietf-mmusic-sdp-dtls-07
o Reference to RFC 7315 replaced with reference to RFC 7345.
Changes from draft-ietf-mmusic-sdp-dtls-06
o Text on restrictions regarding spanning a DTLS association over
multiple transports added.
o Mux category added to IANA Considerations.
o Normative text regarding mux category and source-specific
applicability added.
o Reference to RFC 7315 added.
o Clarified that offerer/answerer that has not been updated to
support this specification will not include the tls-id attribute
in offers and answers.
o Editorial corrections based on WGLC comments from Charles Eckel.
Changes from draft-ietf-mmusic-sdp-dtls-05
o Text on handling offer/answer error conditions added.
Changes from draft-ietf-mmusic-sdp-dtls-04
o Editorial nits fixed based on comments from Paul Kyzivat:
Changes from draft-ietf-mmusic-sdp-dtls-03
o Changes based on comments from Paul Kyzivat:
o - Modification of tls-id attribute section.
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o - Removal of IANA considerations subsection.
o - Making note into normative text in o/a section.
o Changes based on comments from Martin Thompson:
o - Abbreviations section removed.
o - Clarify that a new DTLS association requires a new o/a
transaction.
Changes from draft-ietf-mmusic-sdp-dtls-02
o - Updated RFCs added to boilerplate.
Changes from draft-ietf-mmusic-sdp-dtls-01
o - Annex regarding 'tls-id-id' attribute removed.
o - Additional SDP offer/answer procedures, related to certificates,
added.
o - Updates to RFC 5763 and RFC 7345 added.
o - Transport protocol considerations added.
Changes from draft-ietf-mmusic-sdp-dtls-00
o - SDP 'connection' attribute replaced with new 'tls-id' attribute.
o - IANA Considerations added.
o - E-mail regarding 'tls-id-id' attribute added as Annex.
Changes from draft-holmberg-mmusic-sdp-dtls-01
o - draft-ietf-mmusic version of draft submitted.
o - Draft file name change (sdp-dtls -> dtls-sdp) due to collision
with another expired draft.
o - Clarify that if ufrag in offer is unchanged, it must be
unchanged in associated answer.
o - SIP Considerations section added.
o - Section about multiple SDP fingerprint attributes added.
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Changes from draft-holmberg-mmusic-sdp-dtls-00
o - Editorial changes and clarifications.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
.
[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
DOI 10.17487/RFC4145, September 2005,
.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, .
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, .
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, .
[RFC7345] Holmberg, C., Sedlacek, I., and G. Salgueiro, "UDP
Transport Layer (UDPTL) over Datagram Transport Layer
Security (DTLS)", RFC 7345, DOI 10.17487/RFC7345, August
2014, .
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[RFC8122] Lennox, J. and C. Holmberg, "Connection-Oriented Media
Transport over the Transport Layer Security (TLS) Protocol
in the Session Description Protocol (SDP)", RFC 8122,
DOI 10.17487/RFC8122, March 2017,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
[I-D.ietf-ice-rfc5245bis]
Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", draft-ietf-ice-
rfc5245bis-13 (work in progress), October 2017.
[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-16
(work in progress), December 2016.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-39 (work in progress), August 2017.
14.2. Informative References
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474,
DOI 10.17487/RFC4474, August 2006,
.
[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572,
DOI 10.17487/RFC4572, July 2006,
.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
.
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[RFC6083] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
Transport Layer Security (DTLS) for Stream Control
Transmission Protocol (SCTP)", RFC 6083,
DOI 10.17487/RFC6083, January 2011,
.
[RFC7983] Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
Updates for Secure Real-time Transport Protocol (SRTP)
Extension for Datagram Transport Layer Security (DTLS)",
RFC 7983, DOI 10.17487/RFC7983, September 2016,
.
[I-D.ietf-stir-rfc4474bis]
Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-16
(work in progress), February 2017.
[I-D.ietf-mmusic-ice-sip-sdp]
Petit-Huguenin, M., Keranen, A., and S. Nandakumar,
"Session Description Protocol (SDP) Offer/Answer
procedures for Interactive Connectivity Establishment
(ICE)", draft-ietf-mmusic-ice-sip-sdp-14 (work in
progress), October 2017.
[I-D.ietf-mmusic-sdp-uks]
Thomson, M. and E. Rescorla, "Unknown Key Share Attacks on
uses of Transport Layer Security with the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-uks-00
(work in progress), August 2017.
[ITU.T38.2010]
International Telecommunications Union, "Procedures for
real-time Group 3 facsimile communication over IP
networks", ITU-T Recommendation T.38, September 2010.
Authors' Addresses
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
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Roman Shpount
TurboBridge
4905 Del Ray Avenue, Suite 300
Bethesda, MD 20814
USA
Phone: +1 (240) 292-6632
Email: rshpount@turbobridge.com
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